1. Field of the Invention
This invention relates to integrated circuit fabrication process, and, more particularly, to a process for depositing a thin-film layer of magnetic material onto an insulative dielectric layer of a semiconductor substrate.
2. The Prior State of the Art, and Principal Objects of This Invention
An integrated circuit ("IC") consists of a series of discrete circuit elements which may either be active elements such as transistors or diodes, or passive elements such as resistors or capacitors, which are fabricated in place as an integral part of a semiconductor substrate. Most often the substrate is a silicon "wafer", although germanium and some other types of semiconductive materials have also been used.
Each wafer typically has numerous individual integrated circuits formed on it. Transistors, which are the basic active integrated circuit elements, may be either bipolar, (that is, a transistor which depends on the properties of two types of charge carriers, namely, electrons or "n" type carries and holes or "p" type carriers, for its operation) or unipolar, (that is, one which depends on only one type of charge carrier). Bipolar transistors consist of base, collector and emitter regions formed on a selected layer of the substrate by doping the regions with certain chemical impurities which provide the desired type of charge carriers. Similarly, unipolar transistors consist of gate, source and drain regions formed on a selected layer of the substrate. Typical impurities which can be diffused in silicon to provide "n" or "p" type carrier regions include antimony (n type), arsenic (n type), phosphorus (n type), boron (p type), gallium (p type) and aluminum (p type).
As hereinafter more fully explained, the numerous individual circuit elements of each integrated circuit are formed on the semiconductor substrate using what is commonly called a planar process. Briefly summarized, the planar process consists of forming a passivation layer of oxide or other suitable material on top of the silicon wafer, which is then photolithographically patterned to permit selective introduction of "n" or "p" type impurities into the bulk of the silicon wafer at selected regions, thus altering the electrical properties of those regions. A series of three such selective introductions is required to form isolated bipolar transistors, whereas only one is required for unipolar transistors.
Since the introduction of the concept of semiconductor integrated circuits by Jack Kilby in the late 1950's, and with the development at about this same time of the planar process for making diffused transistors by Robert Noyce and Gordon Moore, semiconductor integrated circuit technology has played an increasingly important role in our society. One of the most fundamental and important applications of semiconductor integrated circuits has been their use in constructing computer memories, and one need only look at the growth of the computer industry to appreciate the degree to which our lives are now affected by this technology.
Prior to the introduction in 1970 of the 1-kilobit, fully decoded random access memory ("RAM"), passive, ferromagnetic storage cells were used as the basic means of storing information in digital computers. This type of storage cell has the inherent advantage of compact construction and lack of power consumption. Even today, passive ferromagnetic-type storage cells are typically preferred in certain types of military applications because they are nonvolatile--that is to say, the information stored in such a cell is not lost in the event of exposure to radiation or temporary loss of power. However, passive ferromagnetic-type storage cells also have a certain disadvantage in that readout of information is destructive since it is erased from the cell during the readout process. A further disadvantage is that the cell provides a very low output signal which must be amplified for proper utilization.
Some designs have been developed so that by using additional electronic circuitry, a "write-after-read" operation can be performed in which the information otherwise erased from the storage cell is immediately written back into the cell after the readout process. However, the use of such electronic circuits increases cost and creates further diminution of the output signal, thus increasing the amplification problem which is already inherent by virtue of the low output signal provided by this type of storage cell.
With the advent and improvement of low cost, high yield manufacturing processes for semiconductor integrated circuits, the cost per bit of semiconductor computer memories has steadily decreased over the last decade. Thus, increasingly the type of cell used to store digital information is derived from the use of active electronic elements arranged on semiconductor ICs in a conventional flip-flop (i.e., cross-coupled transistor) configuration. Active electronic storage cells can be designed using either unipolar or bipolar transistors, which are typically manufactured at a relatively low cost on large or very large scale semiconductor integrated circuits.
These semiconductor or active flip-flop type storage cells have the advantage that readout is nondestructive and the output signal is higher than that of the passive, ferromagnetic-type storage cell. However, an inherent disadvantage is that power is continuously dissipated by the transistor elements, which creates undesirable heating effects. Furthermore, the stored digital information is lost if there is a power failure, even if the power loss is only momentary. The digital information stored in an active storage cell can also be destroyed by high density ionizing radiation and is thus not suitable for those types of applications which require nonvolatile storage.
More recently, it has been proposed to combine the inherent advantages of both passive and active storage elements (i.e., low power consumption and nonvolatility of the ferromagnetic-type storage cell with the nondestructive readout and high signal output of an active flip flop configuration). See, e.g., U.S. Pat. No. 3,573,485 issued Apr. 6, 1971 to Delbert L. Ballard. In this type of combination semiconductor/magnetic storage cell, a circuit is formed using a pair of cross-coupled electronic switching elements such as bipolar or unipolar transistors. Connected with at least one of these active switching elements as an impedance coupler is a passive (i.e., ferromagnetic) storage element. Digital information which is to be stored in the cell is electromagnetically written into the ferromagnetic storage element, thereby changing its effective impedance in the circuit. The readout signal is obtained from the electronic switching elements by providing power to the switching elements in response to a readout address signal, with the electronic switching elements then assuming a conductive or nonconductive state which is determined by the effective circuit impedance provided by the ferromagnetic storage element.
Notwithstanding the inherent advantages of a semiconductor/magnetic-type storage cell, it has been necessary to overcome a number of very difficult practical problems in order to manufacture the passive ferromagnetic element of this type of storage cell as an integrated component of a semiconductive integrated circuit. A principal problem which has been an impediment in this area has been the difficulty in developing a workable process for permanently depositing a thin-film ferromagnetic or magnetic alloy layer into an insulative dielectric layer of a semiconductor substrate.
It is therefore a primary object of the present invention to overcome the problems of the prior state of the art by providing a workable process for depositing a layer of ferromagnetic or magnetic alloy material onto an insulative dielectric layer such that the layer of magnetic material will completely and permanently adhere to the insulative dielectric layer.
Another important object of the present invention is to provide a novel product which includes a thin-film layer of magnetic material deposited so as to completely and permanently adhere to a layer of insulative dielectric material formed on a semiconductor substrate.
The foregoing and other objects and features of the present invention will become more fully apparent from the following description and appended claims, taken in conjunction with the following drawings.